Kidney Week

Abstract: PUB102

A Unique Case of Pseudo-"Normo"natremia

Session Information

• Publication Only

Category: Fluid, Electrolytes, and Acid-Base Disorders

• 1102 Fluid, Electrolyte, and Acid-Base Disorders: Clinical

Author

• Bijjala, Neha, University of California San Diego, La Jolla, California, United States

Neha Bijjala, DO

Introduction

Direct ion-selective electrode (ISE) measurements assess ion activity in undiluted plasma, whereas indirect ISE requires sample dilution and assumes a plasma water fraction of approximately 90%. In states of marked hyperproteinemia or hyperlipidemia, the non-aqueous fraction of plasma is increased. This leads indirect ISE to overestimate the degree of dilution of plasma water, resulting in low sodium measurements (pseudohyponatremia). In contrast, direct ISE measures ion activity in plasma water without dilution and is therefore not affected by excess proteins or lipids.

Case Description

A 67-year-old male with a history of refractory multiple myeloma (progressed despite daratumumab and bortezomib) presented with altered mental status. An extensive neurologic evaluation, including CT head and MRI brain, was unrevealing. The patient was noted to have a persistent primary respiratory alkalosis and ongoing encephalopathy, prompting nephrology consultation for further evaluation of acid–base disturbances.
Laboratory data throughout hospitalization demonstrated significantly elevated total protein levels. Serum sodium values measured via indirect ISE were within the normal range; however, repeat measurement using direct ISE revealed marked hypernatremia. This discrepancy was attributed to pseudonormonatremia in the setting of severe hyperproteinemia.
The patient was treated with aggressive free water administration, resulting in gradual correction of hypernatremia and subsequent improvement in mental status.

Discussion

This case highlights the importance of recognizing pseudohyponatremia and pseudonormonatremia in patients with significantly elevated serum protein levels. In such cases, direct ISE provides a more accurate assessment of true sodium concentration and can uncover clinically significant electrolyte derangements that may otherwise be missed.